Theoretically, nothing is safe from virusses or other malware. You have to keep in mind that things like wii's or PSP's consisist out of million lines of code and if you take into account that on average, there is one big error in every ten thousands lines of code (which is a conservative esstimate!), then it is safe to state that it should certainly be possible to create a virus for the wii or PSP. But it takes much time and effort (and sometimes money) to create a virus, so most virus and malware target popular platforms. I don't think anyone has or ever will create a real virus for the wii. But you never know for certain.
So to summarise: it's safe. probably.